1. Technical Field of the Invention
The present invention relates to a cable steering system in which a steering wheel and a steering gearbox are connected together via a cable such as a Bowden cable.
2. The Prior Art
In a conventional vehicle steering system, a steering shaft has a steering wheel at its upper end and is connected to a steering gearbox at its lower end, and a steering torque that is input into the steering wheel is transmitted via the steering shaft to a rack-and-pinion mechanism disposed within the steering gearbox.
However, when the steering wheel and the steering gearbox are connected together using the steering shaft, since it is difficult to freely choose the position of the steering wheel relative to the position of the steering gearbox, not only are the degrees of freedom of the design greatly restricted, but there is also the problem that the steering gearbox cannot be shared between a right-hand drive vehicle and a left-hand drive vehicle. Moreover, since vibration input into a tire from the road surface and vibration of an engine are input into the steering wheel via the steering shaft, there is the problem that the vibrations might deteriorate the quietness of the passenger compartment and the ride quality.
A cable steering system employing flexible transmission means such as a Bowden cable instead of the conventional steering shaft has therefore been proposed (Japanese Patent Application Laid-open No. 8-2431). Employing such a cable steering system enables the position of the steering wheel relative to the position of the steering gearbox to be freely chosen and, moreover, since it is then difficult for the vibration of the steering gearbox to be transmitted to the steering wheel, each of the above-mentioned problems can be solved.
Furthermore, an electric power steering system is also known that assists a driver""s steering operation by detecting the steering torque that is input into the steering wheel by the driver and driving an assist motor based on the steering torque.
In a cable steering system having such an arrangement, since the cable expands and contracts when a steering torque is transmitted, it is impossible to prevent the transmission rigidity of the steering torque from becoming lower than that of a shaft type steering system in which a steering wheel and a steering gearbox are connected together via a steering shaft. As a result, a time lag occurs between the time when the steering wheel is operated and the time when the vehicle wheels are turned for steering, thereby degrading the steering responsiveness, and there is a possibility that the driver might experience an unpleasant sensation.
In order to solve this problem, if the steering assist power in the power steering system is made larger than usual, the driver is required to input a smaller steering torque into the steering wheel, and there is therefore less expansion and contraction of the Bowden cable, thereby decreasing the steering time lag. However, when this method is employed, the steering torque required of the driver can become too small, resulting in the problem that the steering feel might deteriorate.
The present invention has been carried out in view of such circumstances, so as to prevent any deterioration of the steering feel while enhancing the steering responsiveness in a cable steering system provided with assist means.
In accordance with a first aspect of the present invention, a cable steering system includes a cable that transmits to a steering gearbox a steering torque applied to a steering wheel, steering torque detection means that detects the steering torque, assist means disposed between the cable and a wheel, the assist means assisting a steering operation by a driver, and first control means that controls operation of the assist means based on the steering torque, wherein the system further includes steering reaction force addition means disposed between the steering wheel and the cable, the steering reaction force addition means applying a steering reaction torque to the steering wheel, and second control means that controls operation of the steering reaction force addition means, the steering torque detection means being disposed between the steering wheel and the cable.
In accordance with this arrangement, the first control means controls the operation of the assist means disposed between the cable and the wheel based on the steering torque detected by the steering torque detection means disposed between the steering wheel and the cable, and the second control means controls the operation of the steering reaction force addition means that applies a steering reaction torque to the steering wheel. Therefore, it is possible to prevent deterioration of the steering feel by compensating for a change in the steering torque by generating a steering reaction torque in the steering reaction force addition means while regulating the change in steering responsiveness due to expansion and contraction of the cable by generating an assist torque in the assist means so as to change the steering torque.
Furthermore, in accordance with a second aspect of the present invention, in addition to the arrangement of the first aspect, there is proposed a cable steering system wherein, in a low vehicle speed region, the first control means controls the assist means so as to increase an assist torque and the second control means controls the steering reaction force addition means so as to increase the steering reaction torque and, in a high vehicle speed region, the first control means controls the assist means so as to decrease the assist torque and the second control means controls the steering reaction force addition means so as to decrease the steering reaction torque.
In accordance with this arrangement, in the low vehicle speed region, the assist torque due to the assist means is increased and the steering reaction torque due to the steering reaction force addition means is increased. Therefore, it is possible to prevent the steering wheel from becoming too light while enhancing the steering responsiveness. Furthermore, in the high vehicle speed region, the assist torque due to the assist means is decreased and the steering reaction torque due to the steering reaction force addition means is decreased. Therefore, it is possible to prevent the steering wheel from becoming too heavy while enhancing the handling stability.
Moreover, in accordance with a third aspect of the present invention, in addition to the arrangement of the first aspect or the arrangement of the second aspect, there is proposed a cable steering system wherein, when the vehicle is at a standstill, the steering reaction force addition means assists a steering operation by the driver in cooperation with the assist means.
In accordance with this arrangement, since the steering operation by the driver is assisted by the steering reaction force addition means in cooperation with the assist means when turning the steering wheel when the vehicle is at a standstill, the steering wheel can be made light, thereby alleviating the load imposed on the driver.
Furthermore, in accordance with a fourth aspect of the present invention, in addition to the arrangement of the first aspect, there is proposed a cable steering system wherein the steering torque detection means is disposed between the steering wheel and the steering reaction force addition means.
In accordance with this arrangement, since the steering torque detection means is disposed between the steering wheel and the steering reaction force addition means, variation of the positions at which the steering torque detection means is mounted can be increased.
Moreover, in accordance with a fifth aspect of the present invention, in addition to the arrangement of the first aspect, there is proposed a cable steering system wherein the steering torque detection means is disposed between the steering reaction force addition means and the cable.
In accordance with this arrangement, since the steering torque detection means is disposed between the steering reaction force addition means and the cable, variation of the positions at which the steering torque detection means is mounted can be increased.
Bowden cables 5, 6 of embodiments correspond to the cable of the present invention, a steering reaction force addition motor 17 of the embodiments corresponds to the steering reaction force addition means of the present invention, and a power steering motor 24 of the embodiments corresponds to the assist means of the present invention.